Viscoelastic fluids exhibit some viscous properties also associated with Newtonian fluids, such as water and oil, as well as elastic properties associated with elastic materials, such as rubber. When deformed, viscoelastic fluids resist shear and strain similar to Newtonian fluids. However, unlike Newtonian fluids, the manner in which shear and strain are resisted depends on the rate at which shear and/or strain is applied. That is, for viscoelastic fluids the relationship between stress and strain varies with a duration of time over which a stress or strain is applied, and the rate at which the stress or strain is applied. Examples of time dependent behavior that viscoelastic materials may exhibit, to varying degrees, include increases in strain in response to a stress applied over time (i.e., creep) and/or decreases in stress in response to continually applied strains (i.e., stress relaxation). Some viscoelastic fluids experience shear thinning, which is a decrease in viscosity that accompanies an increase in shear rate.
Viscoelastic fluids are present in a wide variety of applications, including cosmetics, 3D printing, ink jet printing, biological fluid handling (e.g., mucous, sputum), adhesives, and the like. Conventional approaches for forming drops from viscoelastic fluids include the use of tools normally employed for Newtonian fluids, such as agitation, emulsion formation, shear application by valves, and the like. These techniques, when applied to viscoelastic fluids, often result in the formation of long strands that, when pulled, stretch between the drop being formed and the reservoir of material. This phenomenon, often referred to as “beads on a string” phenomena, prevents repeatable formation of fluid drops having desired volumes.